The subject invention relates to a new and improved method for producing an air laid web of hydrogen bonded wood pulp fibers. More particularly, microencapsulated hydrogen bond promoting material is evenly dispersed with wood pulp fibers prior to air laying the mixture on a moving screen. Thereafter, the microcapsules are ruptured enabling hydrogen bonds to form between the wood fibers to produce a consolidated web.
The most widely used prior art methods for forming paper include the production of a slurry which is a combination of wood pulp fibers and water, having a water to fiber ratio in the range of 200 to 1 to 1,000 to 1. In this procedure, the slurry is laid out on a moving screen and suction is used to draw out a portion of the water. Thereafter, rollers are employed in conjunction with various blotting devices in order to squeeze out and remove additional water to produce the final paper product. In this procedure, the presence of water in the slurry is sufficient to produce bonding between the fibers. More specifically, it has been found that wood fibers in the presence of water will form hydrogen bonds between the OH groups of the cellulose molecules in the wood. The resulting paper product, in which the pulp fibers are held together merely by hydrogen bonding, has sufficient strength to be used in a wide variety of applications.
A major shortcoming of the "water laden" processes is that large quantities of water are required to manufacture paper by these techniques. The large quantities of water require very high capital investments per ton of product, severely restricting usable plant locations and in addition, increasing the likelihood of water pollution.
Accordingly, there has recently been developed in the prior art various "dry forming" processes for producing paper without using high volumes of water. In these processes, fibers are air circulated and blown or drawn onto a moving screen to form a three dimensional continuum which then may be pressed into paper. Since there is no slurry to provide the water to form the hydrogen bonding for web consolidation, other means must be provided to consolidate the continuum. For example, when producing high strength paper, such as those used for shipping bags, it has been found that various resins which are sprayed on the surfaces of the paper have an adhesive effect which stabilizes and bonds the web. However, in most paper forming applications, it is unncessary to achieve the high strength produced by using resins. Since the use of resins is relatively expensive, a number of prior art methods have been developed wherein a hydrogen bond promoting material such as water or ammonia is in some way added to the web in order to allow hydrogen bonding to occur between the wood pulp fibers. However, relatively complex and expensive equipment such as spraying devices, are needed to carry out the latter prior art methods. Further, spraying techniques suffer from an additional disadvantage in that only the outer layers of the web are treated, thereby limiting dispersal throughout the web, making complete consolidation difficult to achieve.
One example of a prior art dry forming process can be found in U.S. Pat. No. 3,838,000 issued Sept. 24, 1974 to Urbas. In Urbas there is disclosed a method of producing a dry formed paper product wherein a relatively complex misting chamber is provided in order to spray the surfaces of the web. Another example can be found in U.S. Pat. No. 3,949,035 issued Apr. 6, 1976 to Dunning et al, wherein the surfaces of the moving web are treated with water to induce hydrogren bonding therein. As noted above, both these methods suffer from the disadvantage that water is sprayed only onto the opposed surfaces of the web making complete distribution of the hydrogen bonding throughout the web difficult to achieve.
Another example of a prior art dry forming paper process can be found in U.S. Pat. No. 3,906,064 issued Sept. 16, 1975 to Iannazzi et al. In the latter patent, complex apparatus is disclosed wherein wood pulp fibers are jetted around a conduit in the presence of high humidity in an attempt to disperse moisture through the fibers, prior to their being laid on a moving screen. As can be appreciated, the complex conduit and blowing system disclosed in Iannazzi is relatively complex and therefore expensive.
Accordingly, it is an object of the subject invention to provide a new and improved method of producing an air laid web of wood pulp fibers wherein hydrogen bonding is achieved throughout the entire web.
It is another object of the subject invention to provide a new and improved method for producing an air laid web which is simple and relatively inexpensive to carry out.
It is a further object of the subject invention to provide a new and improved method of producing an air laid web of hydrogen bonded wood pulp fibers wherein micro-encapsulated hydrogen bonding material such as water is evenly distributed with the wood pulp fibers prior to their air laying on a moving screen. The microcapsules are subsequently ruptured to produce a hydrogen bonded web.